The washing machine is a very popular home appliance used to automatically clean clothes. It uses water as its main element. It basically consists of a reservoir which is filled with water, on which the mechanical system shakes the pieces of clothing to be washed. The contemporary machines are manufactured as two basic models, front loading or top loading. The top loading machines receive the clothes in a vertically mounted cylinder, with a central agitator and a top cover. The front loading machines have a horizontally mounted cylinder, with no central agitators, but with a watertight door and with a sight glass.
Both models have the capacity to automatically wash, propelled by an electric motor, conducting washing, rinsing and centrifugation runs. Currently, the use of electronic components replaces complex mechanical systems previously used to control the washing. They control, for example, the water level, one of the main functionalities of washing machines in the current art.
The control by means of electromechanical pressostates, which are devices used to perceive pressures or pressure differences and which, in the case of washing machines, can be translated by differences in the liquids level, is conducted by reading the level indicated by the pressure generated by a water column, which forces a diaphragm, thus activating an electric contact in a single determined point, with the disadvantage of not being able to identify the manometric height and present a continuous reading.
In its turn, the electronic pressostate forwards point-to-point information on the water column height by means of a transducer which varies the output voltage or current according to the pressure applied in the input, allowing a more precise control, admitting, in the most modern machines, the water input control.
The water level control in washing machines is also widely diffused, defining the water volume to be used in washing, upon the user's choice or a previously defined configuration for the equipment, based on a sensor, which is commonly the pressostate, which receives the pressure exerted by the water volume.
Another important event, besides the level control, among the other functionalities introduced in washing machines is recirculation.
Water recirculation is a common feature in washing machines, which is generally intended to homogenize cleaning agents and water, besides increasing the washing efficiency and collecting lints which are released from the clothes and which may affect the equipment performance. In addition, it can minimize the amount of water used for washing through water circulation optimization inside the equipment in certain steps of the washing run.
In order to improve water recirculation during the washing process, perforated baskets were developed, for which the number and size of the holes are dimensioned in order to decrease the total drainage area, thus decreasing the liquid flow from the basket to the tub, causing a delay in the process of balancing the manometric heights upon water recirculation, allowing the accumulation of a decreased amount of liquid in the tub, enabling a lower total water volume to be used in the washing process.
Despite of the recirculation advent, problems related to this functionality can be found as, for example, the excess of foam and high noise level. These problems are generated by air-water mixture, caused by the lack of water input flow control in the basket compared with the output flow from the basket to the tub, in addition to causing a non-uniform flow of the input of liquids into the basket. Without control, water drainage is not uniform when air and water are mixed in the bottom of the tank. This flow range can be significantly decrease when a controllable pump is used.
Currently, due to the mixture of air and water which occurs when the washing machine tub is empty, the use of common detergent generates a lot of foam, thus preventing a complete washing with this type of product from occurring.
Canadian Patent 1112889, according to FIG. 10, describes a washing machine which recirculates water or rinsing water. Its recirculation water input is intended to fully empty the tub, thus generating foam and noise during its operation. The remaining liquid is located in a recess of the tub, forming a pool, in order to prevent the pressostate from indicating that the tub is empty.
Document WO053042, according to FIG. 8, shows a washing machine provided with water recirculation, with the objective to empty the tub during recirculation, however, due to the lack of pump control, it does not maintain a minimum liquid level, producing foam and noise.
Document EP1783264, according to FIG. 9, reports a washing machine equipped with recirculation and a variable speed pump, however, the water level between the tub and the basket remains the same and does not save water
Invention ES8604326 in the public domain, describes the recirculation process, very common in a washing machine, with recirculation controlled by two pressostates and level control elements calibrated for higher and lower level.
Document EP0278461 in public domain, describes the discharge and recirculation flow control process of washing machines based on two pressure sensors adapted to determine the minimum and maximum liquid levels present in the referred machine.